1. Field of the Invention
The present invention relates to a light-frequency control apparatus appropriate for use in, for example, the light source of a frequency multiplex transmission system or the like.
2. Prior Art
Recently, light-frequency control apparatuses which generate CW (continuous wave) light having a pre-specified light frequency have been developed, and an example of the structure thereof will be explained with reference to FIG. 4. In the Figure, reference numeral 1 indicates a reference power source which generates a reference voltage and outputs this as a reference signal. Reference numeral 2 indicates a comparison circuit, which compares the reference signal supplied to one input terminal and a feedback signal (described hereinbelow) supplied to the other input terminal, and generates a difference signal expressing the difference between the signals. Reference numeral 3 indicates a light frequency control circuit, which generates a light frequency control signal in accordance with the difference signal supplied from comparison circuit 2.
Reference numeral 4 indicates an E/O (Electrical signal to Optical signal) conversion circuit, which generates CW light having a light frequency corresponding to the light frequency control signal which is supplied from light frequency control circuit 4. Reference numeral 5 indicates an optical coupler which conducts the CW light outputted from E/O conversion circuit 4 to light output terminal 6, and supplies a portion of the CW light to light frequency discriminator 7. Light frequency discriminator 7 discriminates the light frequency of the CW light outputted from E/O conversion circuit 4 and generates an electrical signal in correspondence with this light frequency, and supplies this to comparison circuit 2 as the feedback signal described above.
In accordance with this type of structure, the CW light which is outputted from E/O conversion circuit 4 is inputted into light frequency discriminator 7 through the medium of optical coupler 5, and is converted into a feedback signal in correspondence with the light frequency of the CW light. Next, comparison circuit 2 compares this feedback signal with the reference signal, and based on the results of the comparison, the light frequency of the E/O conversion circuit 4 is controlled by means of the light frequency control signal which is generated by light frequency control circuit 3; that is to say, the feedback control of the closed loop is conducted.
In the conventional light-frequency control apparatuses described above, in the case in which the closed loop gain was sufficiently large, the degree of frequency stability of tile optical output signal was determined by the operational characteristics of the light frequency discriminator 7. Normally, there were a great number of cases in which a Fabry-Perot resonator was employed in the light frequency discriminator 7. However, Fabry-Perot resonators commonly possesses a plurality of resonance frequencies, so that it was unclear at which resonance point resonance would occur, and there were problems in that the absolute value of the light frequency was unclear, it was impossible to control the light frequencies extending over intervals greater than those between resonance points, and such resonators were susceptible to disturbances such as temperature or the like. Accordingly, in apparatuses employing light frequency discriminator 7 having such defects, there was a problem in that it was impossible to generate stable CW light at freely selected light frequencies.